1. Field of the Invention
This invention relates to optical rotation sensors; and, particularly, this invention relates to ring laser gyros, having an active medium gain that is excited by a radio frequency signal emitted from a helical resonator within an enclosed cavity.
2. Description of the Related Art
Ring laser gyroscopes are a class of optical rotation sensors that have been developed to provide an alternative form of rotational measurement to the mechanical gyroscope. A ring laser gyroscope employs the Sagnac effect to detect rotation. A basic two mode ring laser gyroscope has two independent counter rotating light beams which propagate in an optical ring cavity. These two light beams propagate in a closed loop with transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. Besides the planar ring laser gyro, other path geometries have been used; for example, a non-planar gyroscope has been disclosed in U.S. Pat. No. 4,482,249 to Dorschner, which teaches an out-of-plane light path that provides the reciprocal splitting of two pairs of counter rotating beams. This out-of-plane gyroscope has been known in the literature as the multioscillator ring laser gyroscope.
Yet another alternative to the multioscillator ring laser gyroscope is the split gain multi-mode ring laser gyroscope and method disclosed in a pending patent application to Graham Martin, No. 115,018, filed Oct. 28, 1987, assigned to the common assignee of this application. (Placed under Secrecy Order May 17, 1988). Both the multioscillator ring laser gyroscope and the Split Gain Multi-Mode Ring Laser provide out-of-plane geometry and at least two pairs of counter propogating modes of light beams to measure rotation with respect to an inertial frame of reference. These non-planar gryos have been developed to avoid the need for mechanical dithering. Dithering is needed in planar gyroscopes to prevent counter rotating travelling waves from locking at low rotation rates.
Heretofore, ring laser gyros have operated using at least a dome-like configured metallic or glass covered metallic cathode and at least two anodes, which extend outward from the monolithic glass body of the ring laser gyro to excite the gas medium contained within the gyroscope. A DC discharge has been used which excites gas contained in the ring laser gyro pathway between the cathodes and each of the anodes.
FIG. 1 shows a prior art planar DC-excited ring laser gyro. The ring laser gyroscope 10 is formed from a monolithic glass body such as Zerodur, which is a trademark of the Schott Glass Works Co. of West Germany. A similar glass that may be used as the ring laser gyro 12 is manufactured under the trademark "CERVIT" sold by Owens-Illinois. Both materials are mixtures of glass and ceramic that have opposite temperature expansion coefficients, thus providing an overall minimal dimensional changes over a wide range of temperatures.
A square optical pathway 14 is defined within the gyroscope 10 by 4 legs, 16, 18, 20, and 22. Leg portion 18A, 22, and 20A form a segment of the optical pathway which glows due to the DC discharge between the cathode 24 and the respective anodes 26 and 28. During manufacture, a gaseous mixture of helium and two isotopes of neon provide an active medium that is excited along the DC discharge path defined by segments 18A, 22, and 20A.
Gas is provided to the cavity during manufacture by the fill stem 30 through the anode 26. The cathode is generally grounded, while the anode potentials are each brought up to 1500 volts, through use of a balast resistor 32. At each corner of the pathway 14, a mirror is positioned to reflect light around the ring laser gyro. The mirrors, 34, 36, 38, and 40 are mounted to the frame 12. A more detailed description of the operation of the planar ring laser gyro together with the particular manner of DC excitation is also described in U.S. Pat. Nos. 4,115,004 to Hutchings and 4,612,647 to Norvell, each patent being assigned to the common assignee of this application.
In addition to the high voltage and high current regulation requirements needed for DC excitation, a number of problems have been associated with the manufacture and reliability of DC discharge ring laser gyroscopes. A prime problem is that of Langmuir flow which can cause a biased and therefore inaccuracies in the rotational sensing capabilities of the ring laser gyro, unless the gyroscope is provided with two balanced current discharge paths. A discharge between a single anode and cathode causes the molecules in a gas laser cavity to flow in a preferred direction. This flow gives rise to a bias or inaccuracy in the rotational sensing capability of a ring laser gyroscope, since each of the clockwise and anti-clockwise modes of light beams propogating in the cavity will be influenced differently by this flow phenomena. In a DC discharge excitation mechanism, as illustrated in FIG. 1, the only manner of offsetting the bias problem is to exactly balance the currents and lengths of the two discharge legs 18A and 20A in each half of the discharge region. This is a difficult and costly process. The power supplies associated with DC excitation are expensive and bulky. A 3-4,000 volt potential is necessary to start the discharge processs, and continued operation of the discharge requires a high voltage source of 1500 volts. The cathodes and anodes themselves have associated problems including leaks at the seals and shortened lifetimes.
Also, a phenomenom known as cathode sputtering arises and limits the lifetime of the discharge system. Cathode sputtering is characterized by the degradation of a protective oxide coating on the outside of the cathode for a good part of its life. The discharge process eventually eats through the oxide coating, exposing the underlying aluminum of the cathode. Once this aluminum is exposed, cathode life deteriorates very quickly and results in an inoperative or non-usable laser structure. This cathode sputtering is a severe limitation on the life of a gas ring laser gyroscope. Also there are instabilities in the discharge when the DC discharge is initially activated after filling the ring lasers with gas during manufacture.
In certain ranges of current operation, instabilities in the current and voltage discharge arise. These instabilities limit the range of current and gas pressure that can be used with a DC discharge ring laser gyroscope. Also, the DC discharge process is relatively inefficient in providing high energy electrons to pump the gas laser atomic energy level. Some of the problems associated with DC discharge in a ring laser gyroscope are also described in Laser Applications, edited by Monte Ross, pages 133-200 (Academic Press, 1971).
In addition to the operation of the planar ring laser gyro through DC discharge as shown in FIG. 1, the operation of the multioscillator laser gyroscopes, as described in an article by Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics, vol. QE-16, No. 9, September 1980 is discussed in this article. In both the out-of-plane and Zeeman effect multioscillator ring laser gyroscopes, it is preferred that the active medium not interfere with certain axially uniform fields needed for the operation of these types of ring laser gyroscopes. As with the planar ring laser gyroscope, DC discharge methods have created similar problems for multioscillator ring laser gyroscopes. Likewise, the Split Gain Multi-Mode Ring Laser Gyro, described in U.S. patent application 115,018, filed Oct. 28, 1987, would preferably confine the active medium to the area where a uniform magnetic field is also applied (under Secrecy Order issued May 17, 1988). This is not easily accomplished with DC excitation.
For all the foregoing reasons, an alternative method of excitation of the gain medium of a ring laser gyroscope is desirable.
In the past, alternative methods of excitation of a laser gas medium have been attempted with varying degrees of success. RF excitation of a helium neon mixture has been reported as early as 1961 in Physical Review Letters, vol. 6, No. 3, pages 106-110, in an article by A. Javan. J. P. Goldsborough has described an RF induction excitation of a continuous wave visible laser in vol. 8, No. 6 of Applied Physics Letters, Mar. 15, 1966), pages 137-139.
Smith U.S. Pat. No. 3,772,611, assigned to Bell Telephone Laboratories, issued Nov. 13, 1973, describes an RF excited ring type capallary tube 11 (FIG. 1) which may have utility as a rotation rate sensor. The '611 patent, however, does not teach an efficient design for utilizing the RF excitation. This '611 patent also referred to "A Wave Guide Gas Laser" in an article dated Sep. 1, 1971, in vol. 19, Applied Physics Letters, No. 5, pages 132-134. In this article, Smith described a combined RF and DC voltage excited capillary wave guide containing a mix of helium neon gas.
These inductive coupled RF excitation methods were by necessity high power and created substantial electrical interference and noise which disturbed other instrumentation associated with rotation sensing.
UK patent application published Jul. 19, 1987 (GB 2185846A) discloses a ring laser which is excited by transverse electrical discharge operating at a high frequency alternating voltage. Although this disclosure claims a low voltage excitation range, it operates through capacitive coupling to the gaseous medium in a transverse direction to the passageway between the mirrors. Use of this transverse direction-excited, high frequency, alternating voltage, capacitively coupled to the active medium would result in contamination of the passageways of the ring laser gyroscope cavity due to the constant bombardment of the gaseous media against the walls and the high RF powers needed to drive the discharge. This is counter-productive to a long life operation of a ring laser gyroscope.
Thus, although the prior art referenced have disclosed alternative methods of excitation other than DC discharge for gaseous laser and ring laser gyroscopes, these alternative solutions to the problem of DC discharge have been inadequate.